Support for semipermeable membrane

ABSTRACT

THE INVENTION PROVIDES A CYLINDRICAL RIGID POROUS SUPPORT FOR AN OSMOTIC OR ULTRAFILTRATION MEMBRANE. THE SUPPORT IS MADE OF BRAIDED FIBER SLEEVES, WHICH ARE INTIMATELY CONFORMED TO A MANDREL BY AXIAL LEGTHENING AND CONSEQUENT RADIAL SHRINKAGE OF THE BRAIDED FIBERS AROUND THE MANDREL AND WHICH ARE THEN PERMANENTLY CEMENTED TOGETHER IN SUCH COMFORMED POSITION WITH A LIMITED AMOUNT OF CEMENT TO LEAVE OPEN INTERSTICES BETWEEN THE FIBER CROSSOVER POINTS.

Oct. 3, 1972 HMNES ETAL 3,695,964

SUPPORT FOR SEMIPERMEABLE MEMBRANE Original Filed Oct. 28. 1968 I MWUnited States Patent US. Cl. 156-161 20 Claims ABSTRACT OF THEDISCLOSURE The invention provides a cylindrical rigid porous support foran osmotic or ultrafiltration membrane. The support is made of braidedfiber sleeves, which are intimately conformed to a mandrel by axiallengthening and consequent radial shrinkage of the braided fibers aroundthe mandrel and which are then permanently cemented together in suchconformed position with a limited amount of cement to leave openinterstices between the fiber crossover points.

This application is a division of Ser. No. 770,979, now Pat. No.3,547,272, filed Oct. 28, 1968, a continuation-inpart of co-pendingapplication Ser. No. 508,307, now abandoned, filed Nov. 16, 1965.

The present invention relates to porous supports for semipermeablemembranes of the type used to carry out reverse osmosis orultrafiltration. More particularly, the invention relates to a porouscylindrical support tube having a tubular semipermeable membraneinserted in it as a liner or wrapped about its exterior as an outersleeve.

As is known, when solutions of different concentrations are separated byan ideal semipermeable or osmotic membrane, the pure solvent will passfrom the dilute solution through the membrane into the concentratedsolution until an equilibrium point is reached. At this point, thedifference in solution pressure on opposite sides of the membrane is theosmotic pressure difference between the two solutions. If pressuregreater than this pressure is applied to the concentrated solution, thenthe pure solvent will flow from the concentrated solution through themembrane to the dilute solution and this is known in the art as reverseosmosis. An actual membrane of suitable permeability will pass arelatively greater proportion of solvent than solute during the reverseosmosis flow and, in this way, solvents can be separated from solutions.

Reverse osmosis has specific application to the desalination of sea andbrackish waters and to other separation processes where recovery ofsolvent or more concentrated solution is desired. The solvent recoveryflow rate and the general efficiency of reverse osmosis processes areimproved if a thin semipermeable membrane is used. This requires thatthe membrane be given mechanical support since it is usually veryfragile and incapable by itself of withstanding the reverse osmosispressure to which it is subjected during the process.

Heretofore, various forms of supports for the semipermeable membranehave been proposed, including modified conventional plate and framefilter press assemblies. These have the disadvantage of being costly toconstruct and operate if a reasonable permeation rate is to be achieved.Other proposed forms of supports have been cylindrical pipes or tubesmade of woven or filament wound glass fibers, but such structures havegenerally been incapable of withstanding high reverse osmosis workingpressures on the order of 1,000 to 1,500 p.s.i.

The present invention provides a porous cylindrical sup- 3,695,964Patented Oct. 3, 1972 port tube for semipermeable or osmotic membranes,which avoids the drawbacks mentioned above and provides new and valuableadvantages in the operation of reverse osmosis processes. One of themost important advantages is the fact that the cylindrical poroussupport tubes of the invention can readily withstand reverse osmosisworking pressures of 1,000 psi. and higher with virtually no tendencyfor cracking, bursting or otherwise failing in structural integrity.Since the solvent recovery rate is correlated, in part, diectly to themagnitude of the reverse osmosis working pressure, the greater strengthof the support tubes of the invention permits more rapid and efiicientseparations. Another advantage is the fact that the cylindrical poroussupport tubes of the invention develop the required porosity duringtheir manufacture by an inherent selfadjusting mechanism that isprovided by a critical braided form of interlacing of the reinforcingfibers that are included as part of the tube structure. This, in turn,allows use of self-reacting adhesives which do not shrink or evolvevolatile material during cure, since the required porosity is achievedby mechanical adjustment of the reinforcing fibers and there is no needfor use of an adhesive which becomes porous during its cure. Anotheradvantage is that only a small proportion of adhesive binder is requiredin the final support tube, since the braided fibers or strandsthemselves provide high mechanical strength, even without an adhesive.Thus, the tubes of the invention can be made using an adhesive starvedsystem to obtain adequate porosity, high mechanical strength from thebraided fibers or strands, and a minimum proportion of adhesive which isalways the potentially weakest element in the entire structure. Anotherimportant advantage of the invention is that the braided fibers orstrands included in the support tubes make the tubes capable ofresiliently expanding and contracting to complement the mechanicalsupporting strength provided by the reinforcing fibers themselves,whereby the overall strength of the final tube to withstand the reverseosmosis working pressure is considerably enhanced. Another advantage isthe fact that the porous cylindrical tubes of the invention can bereadily manufactured in commercial quantities with a high degree ofcontrol and precision, whereby desired specifications for the tubes canbe readily duplicated in successive lots or batches.

The foregoing advantages are achieved in accordance with the inventionby use of reinforcing fibers or strands which are in the form of abraided sleeve when assembled as part of the porous cylindrical tubes.These braided sleeves have the characteristic capibility of undergoingconsiderable change in the relationship of diameter to length, that is,as the length of the sleeve is decreased its diameter will increase andas the length is increased the diameter will decrease. This capabilityis utilized in the invention to manufacture support tubes having thegreat strength and other advantages previously described by applyingaxial tension to each layer of sleeving, while it is in place around amandrel, to mechanically conform the sleeving and its reinforcing fibersor strands with high compression to the contour of the mandrel. Morespecifically, in the manufacturing process, at least one braided sleeveis fitted loosely over and around a cylindrical mandrel, the diameter ofthe sleeve being enlarged, if necessary, to achieve this initialmounting by shortening the sleeve length. Then, a tension force isapplied to the loosely-mounted sleeve tending to increase its length,either by pulling its ends, or by anchoring one of its ends and pullingthe opposite end. In reacting to this force, the mounted sleeve willshrink diametrally and thereby all of its braided strands or fibers areinduced to exert a radially inward compression about the mandrelsurface. This, in turn, produces an intimate, close fitting or moldingof the braided fibers or strands to the contour of the mandrel. When alimited proportion of adhesive resin is applied to and cured in theconformed sleeve so as not to fill its interstices but only bond thefiber or strand crossover points, a final tube is obtained havingvirtually the identical shape of the mandrel and with all of the fibersor strands thereof aligned in a perfect orientation as dictated by themechanical increase in length-decrease in diameter force that was usedto conform the sleeve to the mandrel.

While it is possible to produce a porous support tube with a singlelayer of braded sleeve as described above, for best results it isrecommended that a plurality of sleeve layers be used. This gives theadded advantage that each successive sleeve layer, as it is looselymounted and then mechanically conformed by diametral shrinking,reinforces all of the preceding layers so that a thin, dense, compactwall thickness is developed through the thickness of all the layers. Theporosity required for passing solvent by reverse osmosis is retainedeven with a plurality of sleeve layers, since the interstices of eachlayer are oriented randomly both in and out of registry with those ofthe preceding and succeeding layers and application of a limitedproportion of adhesive resin maintains solvent flow paths open throughthe entire wall thickness of the multiple sleeve layers.

With use of two to five layers of braided sleeves, in the mannerdescribed above, final support tubes are achieved which providetremendous self-reinforcement by mechanical interlacement andconformation of fibers or strands to the one and only orientation thatcan be assumed under the force applied to generate axial lengthening anddiametral shrinkage upon the mandrel. It is to be noted that theadhesive resin has nothing to do with developing such orientaton, butonly in maintaining it. Thus, the function of the adhesive is to providesufficient internal bonding and consolidation to permanently retain theorientation mechanically developed in the braided sleeves and,therefore, it need be applied only in a very limited proportion toperform this limited function. This not only allows use of self-curing,non-porous adhesives in economical quantities, but also means that thestresses of reverse osmosis Working pressure are absorbed and withstoodprimarily by the braded strands or fibers, and the operating limits ofthe support tube are a function of the maximum strength of the strandsor fibers, rather than the adhesive. When inherently strong materialssuch as glass fibers are used in the braided sleeves, final sup porttubes of unprecedented strength are achieved capable of operating over along duration under reverse osmosis pressures of up to 1500 p.s.i.,which to applicants knowledge has not been possible before.

The specific form of braiding in the glass'fiber sleeves is notcritical. The only requirement is that some form of braiding be used,and not woven or filament-wound structures which are incapable ofproviding the advantages of the invention previously described.Therefore, the glass fiber sleeve may have any braided form in which oneor more strands cross diagonally and alternately over and under one ormore strands laid in the opposite direction or at an angle.

The adhesive resin may be any waterproof material which will bind thereinforcing fibers and not contaminate the solvent side of the tubeduring use in reverse osmosis. Generally, excellent results have beenachieved with use of epoxy resin adhesives of proper viscosities whichare preferred. Other sealing materials such as polyesters and the likewhich can also be applied in liquid form and then cured to a permanentset are suitable.

The liquid adhesive is applied in excess to the braided stretched sleeveof glass fiber material on the mandrel in any convenient manner, as forexample by brushing or dipping. The saturated fibers are then compressedto wring out the excess liquid resin. This is most conveniently done bya peristaltic force, e.g. by drawing a close-fitting ring downward overthe sleeve, or by rubbing with a sponge or the like to remove as much ofthe liquid adhesive as can be removed by such means. The remainingliquid adhesive tends to collect by reason of surface tension at thevarious points where one strand of fiber crosses another, leaving openpores, substantially free of adhesive in the interstices between them.

The adhesive resin is then hardened. In some cases, the adhesive may beformulated to be self-hardening after a limited period of time. In thecase of epoxy and polyester resins, for example, a polymerizationcatalyst is added to the liquid resin shortly before it is used, and theresin polymerizes to a solid shortly thereafter. Depending on theadhesive system used, it may be necessary to perform an after-treatmentsuch as heating or the like in order to harden it.

Once the adhesive has hardened, the completed support tube is then takenoff the mandrel and is ready for use after any necessary end-fittingsand the like have been added. The adhesive is preferably applied in theform of a solution in a volatile solvent, the particular solventdepending on the adhesive used. The resulting reduction in viscosityadds to the mobility of the adhesive, facilitating its migration to thefiber crossing points and the subsequent removal of excess.

The support tubes according to this invention may be used either asinternal or external supports, but are primarily intended for use asexternal supports, into which a tubular membrane may be inserted as aliner. In either type of use, it is of great advantage that the face ofthe tube in contact with the membrane be as smooth and regular aspossible. When the supporting surface is not smooth, the pressurizedbrine or other solution to be purified may force the membrane to conformto the irregularities of the support, setting up local tensile strainsin the membrane which adversely affect its selectivity and service life.

Therefore, in a preferred embodiment of the invention the inner surfaceof the support tube is made particularly smooth by applying one or morelayers of a smooth liner around the mandrel before applying the glassfiber sleeves forming the principal structure of the support tube. Byway of example, one or more layers of pure, smooth paper, such as finefilter paper, may be pre-moistened with water, or with a dilutepolyvinyl alcohol solution, and Wrapped around the mandrel. The assemblyis then dried to prevent interference of the water with properimpregnation of adhesive. Braided glass fiber sleeves are applied andthe composite assembly impregnated, compressed to remove excessadhesive, and allowed to harden on the mandrel as previously described.After the adhesive has hardened, this smooth liner becomes integral withthe sleeving of the support structure. Other materials may be used inplace of paper. For example, thin sheets of glass fiber matting havealso been used with good results. In fact, any smooth porous material orfinely woven or braided cloth which can be integrally bound to the innersurface of the glass fiber tube by the adhesive used, and does notcontaminate the purified water (and is not adversely affected by it) maybe used.

In cases where the smooth inner liner limits the porosity of thestructure when the sleeve of glass fiber material is several pliesthick, the adhesive may be applied and the excess squeezed out aftereach ply of sleeving is applied to the mandrel or the resin may beapplied and squeezed out once after all the plies of alternatingsleeving are in place. A resin solution containing a relatively largeproportion of solvent can be applied to the first several plies, theexcess squeezed out and the resin hardened, additional plies added andresin containing a relatively smaller proportion of solvent applied tothese, the excess squeezed out and the resin hardened. Such a processmakes for a support tube with a highly porous inner layer and an outerlayer of lower porosity.

Further details of the invention will be readily understood by referenceto the accompanying drawings which illustrate a preferred embodimentthereof and of which:

FIG. 1 is a cross-sectional view showing a mounted assembly of paperliner and stretched braided glass fiber sleeves upon the mandrel;

FIG. 2 is a fragmentary view of a portion of the braided side wall ofthe glass fiber sleeve on a greatly magnified scale prior to mounting ofthe sleeves in the assembly of FIG. 1; and

'FIG. 3 is a view similar to FIG. 2 showing the changes in configurationof the braided side wall of the glass fiber sleeve after it has beendiametrally enlarged during its mounting upon the assembly of FIG. 1.

Referring to FIG. 1 reference numeral denotes a mandrel comprising asolid rod of polytetrafiuoroethylene. A layer of wet filter paper 12 iswrapped around mandrel 10, and overlapped along the length of themandrel 10. After the paper 12 has dried, a first braided sleeve 14 ofglass fiber is slipped over the paper 12 upon the mandrel 10, in theleft to right direction.

Referring to FIG. 2, it will be seen that the braided glass fibers areclosely adjacent each other with little open space between each fiber.This configuration represents the braided glass fiber sleeve 14: priorto being slipped over the mandrel 10.

Referring to FIG. 3, this illustrates the change in the configuration ofthe braided sleeve 14 after its length has been decreased to increaseits diameter to slip it over the mandrel 10. After it is loosely mountedover and around the mandrel 10, the sleeve 14 is pulled taut to increaseits length, whereby the braided sleeve shrinks diametrally and attemptsto return to its original configuration illustrated in FIG. 2. Thisproduces the intimate compression molding effect previously described,to tightly conform the sleeve 14 to the mandrel 10.

Referring again to FIG. 1, three additional braided sleeves 14 of glassfiber are shown mounted successively upon the first sleeve 14, with eachbeing enlarged and diminished diametrally on and around the precedinglayer. This gives an overall assembly of paper liner 12 immediately overmandrel 10, followed by four successive layers of braided sleeves 14.Since all of the layers 14 are pulled taut axially, the entire assemblyis very tightly mounted upon mandrel 10.

Thereafter, a liquid epoxy resin adhesive is applied to the assembly ofFIG. 1, excess adhesive is wiped off and the adhesive is allowed to cureto a permanent set. The mandrel 10 is then withdrawn from theconsolidated rigid assembly of paper liner 12 and braided glass fibersleeves '1-4. The rigid epoxy resin maintains the glass fiber sleeves 14in the optimum conformed orientation to permanently retain the greatstrength of the assembly for withstanding reverse osmosis workingpressure.

In order to further illustrate the nature of the invention and themanner of practising the same, the following examples are presented.

EXAMPLE I A mandrel is prepared by cutting to the desired length apolytetrafluoroethylene (Teflon) rod 0.51 inch in di=a1neter. A singlethickness of fine filter paper is moistened with water and wrappedaround the mandrel, dried and four layers of braided glass sleeving,nominally /2 inch in diameter, are conformed over the mandrel and thepaper line in the manner previously described. The prepared mandrel isthen saturated with an epoxy-polyamidesolvent formulation consisting of:

Epoxy resin (ERL 2795): 60 parts (wt.) Polyamide (Versamid 125): 40parts Trichloroethylene: 47 parts The saturated lay-up is then subjectedto peristaltic action by wiping it down with a sponge compressed aroundthe circumference of the glass sleeving, to remove as much as possibleof the impregnated adhesive. The solvent is then evaporated and theformulation is cured in an oven at 212 F. for two hours, after which thecompleted porous support tube is easily removed from the mandrel.

EXAMPLE II A mandrel is prepared by placing a 34 inch length of /2 inchOD x inch ID Teflon tubing on a inch steel drill rod 3 feet in length.On one end of the steel rod is threaded a round stop nut /2 inch 0D. Theother end of the steel rod is left bare for gripping.

A 2.1 inch by 22 /2 inch strip of Whatman No. l filter paper is cut froma sheet 22 /2 by 18%. inch.

The strip of filter paper to serve as liner is wet with water andcarefully placed on the Teflon mandrel and shaped to conform to it withabout /2 inch overlap. It is then left to dry.

With one end of the steel rod gripped in a vice, a 2 /2 foot length ofbraided glass sleeving /2 inch diamter (style E6 530, Electra Insulation'Corp.) is slipped over the filter paper, taped at the free end (i.e.,the end that is not in the vice) with an adhesive tape, and then pulledtaut. This is repeated for a total of five layers of braided sleeving,pulling each taut to tightly conform the layers to the mandrel.

The lay-up (consisting of the liner and layers of reinforcement) is thenremoved from the vice and placed on a sheet of polyethylene film.

Anf impregnating formulation is next prepared consistmg 0 Parts ERL 2795(Union Carbide) 6 Versamid (General Mills) 4 Trichloroethylene 40 Thisis mixed thoroughly.

The impregnating formulation is poured over the layup and allowed tosoak for five minutes while covered with a plastic sheet to preventevaporation of the solvent.

The lay-up is then placed vertically and the excess resin is squeezedout by wrapping a paper towel around the top end (the end at which theglass sleeving had been taped), grasping it tightly while drawingdownward. This milking procedure is repeated twice.

The finished lay-up is placed in a hot air circulated oven at 210 F. fortwo hours in order to cure.

The lay-up is removed from the oven and allowed to cool. The mandrel isnext pulled out while the stop nuts prevents the Teflon from coming olfthe steel rod. The resulting porous tube is then cut to the desiredlength.

The procedure as just described may be varied in many respects, as willbe apparent to those skilled in the art. For instance, the mandreldimensions may be varied, and other materials of construction may beused. An all metal mandrel may be employed, if it is first coated with amold release agent.

Other materials may be used for the liner, such as writing paper, glassfiber paper and many other materials. With some of these, the step ofwetting the liner prior to placing it on the mandrel may be omitted.

The impregnating formulation may vary widely. For example, other epoxyresins, polyester resins, polyurethane resins and other solvents such astoluene, xylene, methylisobutyl ketone, acetone, benzene, methylethylketone, perchloroethylene, methyl alcohol, isopropyl alcohol,chloroform, 1,1,2-trichloroethane, carbon tetrachloride, and others maybe used, as well as mixtures.

The method of impregnating the coating may be varied, for example, bybrushing or spray-coating.

The removal of excess resin can be effected in a variety of other ways,for example with string or by using a circular squeegee.

The curing time and temperature will vary greatly depending on the resinsystem, from room temperature to a few hundred degrees and from minutesto days.

In some applications, it is advantageous to employ a plurality of nestedsupports. Thus, a first support tube may be prepared, having arelatively high porosity and this may be inserted into a second tube,separately prepared, the second tube having a relatively low porosity.The osmotic membrane is then inserted as a liner in the first or innersupport tube.

While this invention has been described in terms of certain preferredembodiments and illustrated by way of certain drawings and examples,these are illustrative only, as many alternatives and equivalents willreadily occur to those skilled in the art, without departing from thespirit and proper scope of the invention.

What is claimed is:

1. [A method of making a porous tubular support for a semipermeablemembrane which comprises the steps of placing at least one layer ofbraided strands of fibrous material around a tubular mandrel, thebraided fibrous material having a characteristic capability ofundergoing a change in relationship of diameter to length, inverselychanging the relationship of diameter to length of said material toconform the material to said mandrel; saturating said layer with aliquid adhesive material in an amount sufiicient to consolidate saidstrands into a rigid structure, but insufficient to fill the intersticesbetween said braided strands, hardening said adhesive and removing theresultant support from said mandrel.

2. A method as in claim 1 wherein said fibrous material is glass fiber.

3. A method as in claim 1 further comprising the step of placing asmooth liner around said mandrel prior to placing said layer of braidedstrands of fibrous material around said mandrel.

4. A method as in claim 3, wherein said liner material comprises a layerof cloth.

5. A method as in claim 3 wherein said liner material comprises a layerof paper.

6. A method as in claim 1 wherein said saturating step includes the stepof eliminating excess adhesive by compressing and wiping said braidedstrands.

7. A method as in claim 1 wherein said adhesive material is a reactiveepoxy resin.

8. A method as in claim 1 wherein said adhesive material is a polyesterresin.

9. A method as in claim 1 wherein a plurality of braided sleeves ofglass fiber material are placed around said mandrel, each of saidsleeves being pulled axially to compress it around said mandrel bydiametral shrinkage thereof, said plurality of stretched sleeves beingplaced successively around said mandrel concentrically with respect toeach other.

10. A method of making a porous tubular support for a semipermeablemembrane which comprises the steps of placing at least one layer ofbraided strands of fibrous material around a tubular mandrel, axiallypulling said layer to compress it circumferentially to conform to saidmandrel, saturating said layer with a dilute liquid adhesive material inan amount sufiicient to consolidate said strands into a rigid structure,but insufiicient to fill the interstices between said braided strands,placing at least one additional layer of braided strands of fibrousmaterial around said first layer, axially pulling said second layer tocompress it circumferentially to conform to said first layer, saturatingsaid second layer with a more concen trated liquid adhesive material inan amount sufiicient to consolidate said strands into a rigid structurebut insufl'icient to fill the interstices between said braided strands,hardening said adhesives and removing the resultant support from saidmandrel.

11. A method as in claim 1 wherein said tubular mandrel comprises anexterior surface of polytetrafiuoroethylene resin.

12. A method as in claim 1 which includes the step of inserting anosmotic membrane within the porous tubular support, thereby providing aporous tubular supported semipermeable membrane.

13. A method as in claim 1 which includes preparing a first poroustubular support having a relatively high porosity and inserting saidfirst tubular support into a second porous tubular support having arelatively low porosity.

14. A method as in claim 3 which includes wetting the liner and shapingthe wet liner to conform to the contours of the mandrel.

15. A method as in claim 3 which includes bonding the liner to the innersurface of the porous tubular support.

16. A method as in claim 10 which includes placing a smooth liner aroundsaid mandrel prior to placing said first layer of braided strands offibrous material around said mandrel.

17. A method as in claim 10 wherein said fibrous material is glassfiber.

18. A method as in claim 10 which includes a step of inserting anosmotic membrane within the porous tubular support, thereby providing aporous tubular supported semipermeable membrane.

19. A method of making a porous tubular support for a semipermeablemembrane, which method comprises the steps of:

(a) placing a linear material about a tubular mandrel;

(b) placing a plurality of braided sleeves of glass fiber materialaround said mandrel;

(c) axially pulling each of said sleeves to compress the sleevescircumferentially to conform said sleeve to the mandrel;

(d) saturating each sleeve so compressed with a liquid adhesive materialin an amount sufiicient to consolidate the strands into a rigidstructure, but insufiicient to fill the interstices between the braidedstrands, thereby maintaining a solvent flow path through the entiretubular support so prepared; and

(e) hardening the adhesive material and removing the resulting supportfrom the mandrel, thereby providing a porous multilayered tubularsupport for a semi permeable membrane comprising an internal liner and aplurality of concentric braided glass fiber rigid strands.

20. A method as in claim 1 which includes axially pulling said layer tochange its length-to-diameter ratio and to compress it circumferentiallyto conform to said man= drel.

References Cited UNITED STATES PATENTS 869,686 10/1907 Bauno 46-1 R2,864,506 12/1958 Hiskey 210-22 X 2,936,257 5/1960 Nailler et al 156l60X 3,399,092 8/1968 Adams et al l56173 X 3,474,703 10/1969 Davis et al156l48 X 3,490,975 1/1970 Lightwood et al. l56175 X BENJAMIN R. PADGETT,Primary Examiner E. LEHMANN, Assistant Examiner US. Cl. X.R.

